This invention relates to an apparatus for detecting and measuring the pressure under which the air-fuel mixture is compressed during the compressed stroke, and more particularly to an apparatus of the type which is capable of detecting and measuring a compression pressure that is produced during the compression stroke to compress the air-fuel mixture in the combustion chamber.
It is well known that the internal-combustion engine provides its very good operating characteristics by compressing the air-fuel mixture to a desired compression ratio. It is also known that if there is any leakage of the compression pressure from the combustion chamber with an accompanying reduction in the pressure during the compression stroke, it may appreciably impair the efficiency of the engine which leads to a decrease in the torque or misfire.
In order to avoid such abnormal situation and in order to run the engine under the most satisfactory condition, it is desirable to previously measure compression pressures exerted in the combustion chamber. From this aspect, it is the practice that the compression pressure is measured as often as possible during the assembly line or periodical inspection and maintenance of the engine in attempts to run the engine in the optimum condition.
There is known a compression tester which is generally used for this purpose. In the conventional practice according to the tester, spark plugs are usually removed from the combustion chamber. The tester has a member from which the pressure is to be admitted into the tester by inserting the member into the combustion chamber. The tester has also an indicator which indicates the pressure under which the air-fuel mixture is compressed in the combustion chamber by actuating the bellows or diaphragm under the pressure that has been admitted. In other words, the method according to the tester consists of steps of removing spark plugs from the engine, of inserting a part of the tester into the combustion chamber which must be carefully kept airtight after removal of the spark plugs and of running the engine by means of a cell-driven motor, for example, so as to compress the mixture in the combustion chamber for measuring the pressure thus exerted. However, this presents disadvantages from the aspects of the operability and precision that the tester provides, since it always requires removal of spark plugs from the combustion chamber, and great care must be taken to keep the chamber airtight for insertion of the tester therein. For this reason, significant measuring errors may occur, depending on the manner in which the tester is disposed.
There is another known method of measuring the compression wherein a pressure transducer incorporating a semiconductor strain gauge is employed in lieu of the earlier mentioned tester. However, this method is disadvantageous alike since it requires a transducer of a relatively complicated construction.
An apparatus provided according to the invention is intended for measuring compression pressures exerted in the combustion chamber by detecting and analysing secondary voltages of the ignition system. More specifically, the present invention is based on the observation that a secondary voltage includes a portion which has a value corresponding to the level at which a compression pressure is exerted during the compression stroke in the combustion chamber. The experimental results have also shown that the portion of induced discharge voltage contained in the secondary voltage changes proportionally with the level of the compression pressure. It will be understood from the above that the level of the compression thus exerted can be easily and visibly obtained by detecting the secondary voltage and analysing the waveform thereof.